1. Field of the Invention
This invention relates to an image display device which utilizes the electrochemical color forming and bleaching phenomenon. More particularly, it is concerned with an image display device which utilizes an improved electrically responsive color forming and bleaching medium to prolong its durability against repetitive uses.
The term "electrochemical color forming and bleaching phenomenon" is meant by a reversible color forming and bleaching phenomenon dependent on the electric polarity, in which, generally speaking, a certain element forms color depending on its electric conduction, and bleaches the color (i.e., reinstatement to the original color by its electric conduction in an opposite polarity to that for the color forming, or by heat application, or by a combination of these treatments. This is usually called "electrochromic phenomenon".
Mechanism for causing the electrochromic phenomenon is not always simple, although, in most cases, it is considered due to the so-called oxidation-reduction reactions between an electrolyte and a color forming substance. In this case, there can be made no clear distinction between the electrolyte and the color forming substance in their material aspect, i.e., one and the same substance may, in some cases, be the color forming substance as well as the electrolyte. From another standpoint, this electrochromic phenomenon is understood to take place by variations in the light absorbing characteristic of such substance due to injection of electrons into the center of color element. In reality, however, such phenomenon is considered to occur as the result of combination of the oxidation-reduction reactions and the electron injection.
Since the electrochromic phenomenon is to cause color which a material possesses primarily to change electrically, the color combination is of wide varieties. Also, whether the substance can permit light to pass therethrough, or reflect or scatter it is not determined by the property of the substance per se, but by a method of forming a layer of such substance. Therefore, in the case of using such substance as the display element, it should have such a property that it may be formed either in the light transmission type or in the light reflection type.
Among various kinds of such electrochromic phenomenon, a phenomenon based on the electrolysis of the substance due to electric current, i.e., oxidation and reduction reactions of the substance (redox reaction) is especially called "electrochemichromism", the study of which is now in fashion in the field of image display technology.
In general, for the display device, the technology of which is cased on the electrochemichromism, there has been known to use a cell which accommodates therein a working electrode and a counter electrode having a light transmitting or reflecting property, and an electrically responsive chromic substance consisting of an electrochemical color forming and bleaching substance, an electrolyte, and a solvent capable of dissolving these substances. This electrochemical color forming and bleaching substance reversibly changes its oxidation and reduction conditions by passage therethrough of electric current to be able to bring about a detectable change on the outer appearance of the working electrode, i.e., the reduction in the substance causes the color forming, while the oxidation therein causes the color bleaching, or vice versa. These two electrodes and the electrolytic solution are accommodated in an appropriate housing having means, through which the working electrode can be seen. The electrochemical color forming and bleaching substance in such image display device is capable of accepting or donating electrons, by which it usually turns into a radical ion having a high degree of light absorption in the visible range of the spectrum, and, at the same time, this radical ion combines with an anion existing in the medium to form on the working electrode a color forming body insoluble in the medium.
The above-described cell for the image display device is driven in three stages, i.e., (1) a write-in operation to the working electrode, which takes place when a direct current voltage is applied thereto from outside so that the working electrode may be in the negative polarity and the counter electrode in the positive polarity; (2) a memory operation which takes place when the external voltage application is interrupted and the circuit is opened (the write-in operation on the working electrode being continued); and (3) an erasing operation of the contents written in on the working electrode by application of a direct current voltage from outside in a manner to render the working electrode to assume the positive polarity, and the counter electrode the negative polarity. Incidentally, it is also possible to effect the write-in operation with the working electrode being in the positive polarity.
2. Description of Prior Arts
U.S. Pat. No. 3,712,709 describes the image display device of the above-described type, in which the color forming and bleaching medium consisting of N,N'-di(p-cyanophenyl)-4,4'-bipyridinium salt, potassium chloride, sodium ferrocyanide, diluted sulfuric acid, etc., is used.
U.S. Pat. No. 3,806,229 describes an image display apparatus, wherein a color forming and bleaching medium consisting of salts of dipyridinium compounds, and an adjuvant such as substituted hydroquinones having a standard oxidation-reduction potentials of 0.7 v and above, ferrous salts, or 1,4-di(dialkylamino)benzenes, etc, is held between opposing electrodes.
Further, U.S. Pat. No. 3,930,717 discloses a similar type of image display device.
In either of the abovementioned image display devices as taught in the prior patents, however, there has been pointed out that repetitive durability of the image display element, i.e., its lifetime, constitutes a problem. In more detail, such phenomena as insufficiency in color forming, insufficiency in color bleaching, occurrence of side-reaction in the color forming, occurrence of irregularity in the formed color, changes in color tone, etc. remarkably curtail the lifetime of the image display element. The main cause for such shortened service life of the element is presumed to be electrode contamination. The contamination is said to be caused by various factors such as impurities contained in the electrochemical color forming and bleaching substance, products from chemical changes in such electrochemical color forming and bleaching substance, impurities discharged from the cell container, inadequacy in the driving system, and others, all these factors being combined sophisticatedly.
To improve such disadvantages, there have been proposed improved techniques concerning new adjuvants or auxiliary agents to be added to the color forming and bleaching substance, or the color forming and bleaching medium.
For example, U.S. Pat. No. 4,018,508 discloses an image display device which provides the most highly improved service life. In this image display device, there is used the color forming and bleaching medium consisting of N-(p-cyanophenyl) substituted derivatives which are bicyclic compounds having two nitrogen containing rings, an auxiliary redox system in the form of Fe.sup.++ .revreaction.Fe.sup.+++, and a carboxylic acid complexing agent. The auxiliary redox system is used for reversibly effecting the electrochemical oxidation-reduction reactions of the color forming and bleaching substance with good balance, while the carboxylic acid complexing agents is used for preventing Fe.sup.+++ ion resulted from the reaction in the auxiliary redox system from causing sedimentation of an insoluble Fe(OH).sub.3. The patent specification describes that, for the carboxylic acid complexing agent, saturated mono-oxycarboxylic acid and dioxycarboxylic acid, particularly, .alpha.- and .beta.-OH carboxylic acids are preferred. As the concrete examples, there are enumerated in the patent specification: formic acid, acetic acid, citric acid, tartaric acid, succinic acid, malic acid, malonic acid, cyclopropane-1,2-dicarboxylic acid, and cyclopropane-1-OH,2-carboxylic acid.
Furthermore, this patented device controls the pH value of the color forming and bleaching medium to the side of a relatively strong acidity (i.e., pH=1 to 6, or preferably pH=1 to 4, or more preferably Ph=2 to 3) with a view to stabilizing the reaction of the auxiliary redox system (Fe.sup.++.revreaction.Fe.sup.+++).
Even in such improved image display device as described in the foregoing, however, there still remain difficult problems which prevent the device from being put into practical use. Such difficult problems are, for example: that reversibility in the oxidation-reduction reactions of the color forming and bleaching substance cannot be satisfactorily established, even when the auxiliary redox system performing the reaction of Fe.sup.++ .revreaction.Fe.sup.+++ is used, hence insufficiency in the color forming and color bleaching, the side reaction in the color forming, and so forth tend to occur; that sedimentation of Fe(OH).sub.3 cannot be prevented, even if the carboxylic acid complexing agent is used, hence irregularity in the formed color would take place on the surface of the electrodes; that deterioration in the electrode material is apt to take place when the pH value of the color forming and bleaching medium is brought to the acid side (this tendency is particularly remarkable when an oxide electrode such as SnO.sub.2, In.sub.2 O.sub.3, etc. is used); and various other problems. On account of such various problems still to be solved, the situation as at present is that no image display device having satisfactory repetitive durability has yet been realized.